Review and discuss the significance of animal cloning (such as: Dolly the sheep)

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Review and discuss the significance of animal cloning (such as: Dolly the sheep)

Animal Cloning is the production of one or more identical copies of a genome of a particular animal. Embryo splitting (or blastomere separation) and nuclear transfer are two methods by which animals may be cloned. Both of these methods require gestation in the uterus to attain complete development. Embryo splitting is the process whereby an embryo has its cells (blastomeres) separated soon after fertilization at the 2nd, 4th or 8th cell stage. At this stage, each cell is considered to be at a stage where the genetic material has not been programmed to develop into a select tissue, it is undifferentiated and can therefore develop into a variety of tissues, or a complete organism given the correct conditions. This is referred to as totipotency and it allows scientists to split animal embryos into several cells to produce multiple organisms that are genetically identical.

Modern techniques begin with stripping the embryo of the protective layer. After each blastomere has been separated from the embryo mass, the cell is encased in its own protective synthetic layer. Each blastomere cell, is now considered a new separate embryo and is cultured in vitro and later in vivo in a surrogate mother until birth (Roberge, 2004).

Cloning by nuclear transfer is based on the concept that the animal’s genome is located in the cell nucleus. The only exception to this is the small amount of DNA of 16, 000 base pairs found in the mitochondria. In this process the genetic material from one cell is place into a recipient unfertilized egg which had its genetic material removed by enucleation. It is then necessary to activate the cell to start dividing. In mammals the egg must be artificially placed in the womb of a surrogate mother where it will grow until birth. During the mid 1980s, the first mammals were cloned by this process, thirty years after the initial experiments with frogs (Van Eenennam, n.d). Many reports of successful nuclear transfer experiments in mammals have been made, nearly all of them using cells taken directly from early embryos.  

Two types of nuclear transfer that are used to clone animals are the Roslin Technique and the Honolulu technique. The Roslin technique is a variation of the somatic nuclear transfer, and this method was used to create Dolly. In this process, somatic cells with their nuclei are allowed to grow and divide, these cells are then deprived of nutrients to induce the cells into a suspended or dormant stage. An egg cell that has had its nucleus removed is then placed in close proximity to a somatic cell and both cells are shocked with an electrical pulse. The cells fuse and the egg is allowed to develop into an embryo.

The Honolulu Technique was developed at the University of Hawaii by Dr. Teruhiko. In this method, the donor nucleus from a somatic cell is removed using a special pipette, and is microinjected into an egg that has had the nucleus removed. The egg is placed in a chemical solution and it is cultured. The developing embryo is then implanted into a surrogate and allowed to develop. Twenty-two fertile, female mice were cloned from nuclei of adult ovarian cumulus cells by this method.

Dolly the sheep was the first mammal to be cloned from an adult cell. She was derived from cells that were taken from the udder of a six year old ewe. In this case, the genetic material in the nucleus was transferred from adult somatic cells that were cultured. Individual cells were then fused with unfertilised eggs from which the genetic material had been removed. Two hundred and seventy seven of these 'reconstructed eggs' were cultured for 6 days in temporary recipients. Twenty-nine of the eggs that appeared to have developed normally to the blastocyst stage were implanted into 13 surrogate Scottish Blackface ewes. One gave rise to a live lamb, Dolly, 148 days later.

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The Nuclear transfer technique was first reported in frogs in 1952 and has been used widely since in amphibians to study early development. In 1977, the cloning of mice using nuclei from early embryos was reported, however this work was not repeatable. Research on nuclear transfer however continued in cattle, this was driven by the prospect of large commercial benefits from multiplying elite embryos.

In the early 1980’s the former Animal Breeding Research Organisation started research aimed at producing transgenic sheep and cattle that would secrete human proteins in their milk.  By the middle of the 1980’s the number of ...

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